The present invention relates to a brake-pressure control device for a road vehicle which is equipped with devices for anti-lock control (ABS), for electronically controlled brake-force distribution (EBKV) and for drive-slip control (ASR). More particularly, the control device includes brake-pressure regulating members comprising hydraulic regulating cylinders and electromotively drivable pistons therein configured to be activated by output signals from an electronic control unit to control brake-pressure generation in at least individual wheel brakes of the road vehicle in the event of driver-controlled braking and to obtain, independently of an actuation of the brake system by a vehicle driver, a loading of the wheel brake of a driven vehicle wheel tending to spin with brake pressure, necessary for a drive-slip control. In a braking operation controlled by the vehicle driver, an electronic control of front-axle/rear-axle brake force distribution occurs. With anti-lock control on the individual wheels, brake-pressure reduction and brake-pressure rebuild-up phases are controlled, the pistons of at least individual regulating members having as a basic position a middle position, out of which is achieved both a pressure build-up stroke of the respective piston up to the maximum brake-pressure level and, starting from this level, a pressure-reduction stroke down to full pressure relief. The regulating members are equipped with a position transmitter recording a position of the associated piston, and electrically activatable inlet valves are provided for selectively connecting and disconnecting at least individual wheel brakes directly to a pressure outlet of the brake unit.
A brake-pressure control device is described in DE-4,103,783 C2 and comprises, in addition to a driver-actuatable brake unit of the hydraulic multi-circuit brake system for brake-pressure generation, brake-pressure regulating members which are configured as hydraulic regulating cylinders with electromotively drivable pistons. As a result of the activation of the regulating members by way of output signals from an electronic control unit, brake-pressure generation can be controlled in at least individual wheel brakes of the vehicle in the event of driver-controlled braking and, independently of an actuation of the brake system by the driver, the loading with brake pressure of the wheel brake of a driven vehicle wheel tending to spin, necessary for drive-slip control, can be achieved.
In the event of braking which the driver controls with the known system, an electronic control of the front-axle/rear-axle brake-force distribution can be achieved by the brake-pressure regulating members. In the event of anti-lock control on the individual wheels, brake-pressure reduction and brake-pressure rebuild-up phases can be controlled by the brake-pressure regulating members, with the pistons of at least some of the regulating cylinders having as a basic position a middle position, out of which both a pressure build-up stroke up to the maximum brake-pressure level and, starting from this level, a pressure-reducing stroke down to full pressure relief can be achieved. These regulating cylinders are equipped with a position transmitter recording the piston position. There are also provided electrically actuatable inlet valves, by way of which at least some of the wheel brakes can be connected directly to the pressure outlet of the brake unit and can be shut off relative thereto.
In the known brake-pressure control device, the position-monitored brake-pressure regulating members are assigned to the driven rear wheels of the vehicle. The pistons of the pressure modulators assigned individually to the front-wheel brakes have a basic position in which the volumes of the outlet-pressure spaces of these pressure modulators are minimal. In these basic piston positions, inlet valves controlled mechanically thereby are maintained in their open basic position, in which the connected wheel brakes can be loaded with a brake pressure. When the brake-pressure regulating members are actuated with the effect of enlarging their outlet-pressure spaces, after a brief initial portion of the pressure-reducing stroke of the regulating-member pistons, the inlet valves change over to their closing positions, after which, as a result of further piston displacement, corresponding pressure reduction in the particular connected wheel brake is achieved.
An automatic actuation of one or both front-wheel brakes, for example for the purpose of regulating the driving dynamics with the effect of achieving a desired steering behavior of the vehicle, is not possible with the known brake-pressure regulating device. It would, if appropriate, be possible to consider utilizing the brake-pressure regulating members, provided for the rear-wheel brakes for the purpose of drive-slip and anti-lock control, with the neutral middle position of the pistons, also for the front-wheel brakes and, during the normal operation of the brake system, (that is to say an operation not subjected to anti-lock control), feeding the outlet pressure of the brake unit into the wheel brakes via the outlet-pressure spaces of the brake-pressure regulating members.
However, with such a possible modification of the known brake-pressure regulating device, the modification being suitable theoretically for regulating the driving dynamics, a highly unfavorable pedal characteristic would occur in the normal braking mode. That is, on account of the large volumes of the outlet-pressure spaces of the brake-pressure spaces of the brake unit, relatively large-volume sealing rings or sealing sleeves would also be required for sealing off the pistons of the brake-pressure regulating members relative to their housings. These sealing rings or sealing sleeves would also be deformed increasingly under an increasing pressure and consequently lead to a "flexibility", i.e., a soft reaction, of the brake pedal. As a result, there would also be an increase in the pedal travel which not only would be felt as extremely uncomfortable, but is also undesirable for ergonomic reasons, because the actuating angle, within which the driver can generate his highest actuating force, is narrowly restricted, and therefore long pedal travels are also disadvantageous for safety reasons.
If, on the other hand, pressure modulators of the type described in DE 4,103,783 C2, in which the basic position of the modulator pistons corresponds to the largest volume of the outlet-pressure spaces, were used as brake-pressure regulating members for the front-wheel brakes, then, in order to guarantee anti-lock control, these modulators would also have to be utilized for the normal braking mode and therefore be shut off relative to the brake unit. This has the disadvantageous result of the brake pedal then being unfavorably "hard" and a pedal-travel simulator would therefore be necessary. In turn, this has the further disadvantageous result that the brake-pedal reaction would be determined solely by the travel simulator, for example a compressible spring, and would be, at most, an approximation of the brake pressures.
Travel simulators with this function are provided, for example, in the brake-pressure control device described in DE 4,102,497 C1, which effects automatically controlled braking when the driver actuates the brake pedal at a speed .o slashed. which is higher than a relevant threshold value .o slashed..sub.S, which an electronic control unit interprets as an indication that the driver would like to initiate full braking with high vehicle deceleration. In such a situation, the brake circuits in the known brake-pressure control device are shut off relative to the pressure outlets of a tandem master cylinder assigned thereto and, instead, there are connected to these pressure outlets buffer accumulators, into which brake fluid can be forced out of the master cylinder even under moderate actuating force, so that the driver's wish, which is recorded by a pedal-position transmitter, continues to remain recognizable.
In the known brake-pressure control device, the brake-pressure generation takes place, during automatically controlled full braking, by pressure modulators which are assigned individually to the brake circuits and are activated hydraulically by the outlet pressure of a high-pressure pump and the outlet pressure of which is metered to the brake circuits via metering valves actuatable in a pulsed manner. Inserted between these pressure outlets of the pressure modulators and the wheel brakes is the hydraulic unit of a standard anti-lock system which, if appropriate, is also utilized for carrying out a drive-slip control. In this further known brake-pressure control device, the buffer accumulators and the control valves necessary for coupling them .to the brake unit are required purely for automatic full braking, thus to that extent implying considerable technical outlay.
Proceeding from a known brake-pressure control device, therefore, an object of the invention is to improve such a brake-pressure control device so that, on one hand, it allows, without appreciable extra outlay, further possibilities of regulation and/or control, such as, for example, control of driving dynamics and/or, if necessary, automatically controlled full braking; on the other hand, in normal braking operation, that is to say operation not subjected to anti-lock control, it allows a sensitive reaction behavior of the brake pedal and the generation of high brake forces, along with a nevertheless "short" ergonomically favorable pedal travel.
This object has been achieved according to the present invention by providing that the brake-pressure regulating members, at least for the front-wheel brakes, are configured such that the pistons have a basic position out of which brake-pressure build-up and reduction strokes can be executed. Brake-pressure regulating members for the rear-wheel brakes are configured such that a basic position of the pistons corresponds to the largest volume of the outlet-pressure spaces of the brake-pressure regulation members. In normal braking, i.e. braking not subjected to anti-lock control, these brake-pressure regulating members are also utilizable for brake-pressure generation and are shut off relative to the brake unit of the brake system.
According to another aspect of the present invention, at least one sensor is configured to generate an output signal which is fed to the electronic control unit and which, when braking is initiated and controlled by the vehicle driver, is a measure of expected value of the vehicle deceleration. From comparative processing of the sensed signals characteristic of the piston positions of the brake-pressure regulating members of the front-wheel brakes with the sensed signals characteristic of the expected value of the vehicle deceleration, the electronic control unit is operable to generate activating signals for the drives of the regulating members of the front-wheel brakes to cause the pistons to move into the position assigned to the expected value and also activating signals for the brake-pressure build-up operation of the brake-pressure regulating members of the rear-wheel brakes.
Accordingly, for the front-wheel brakes brake-pressure regulating members, the pistons of which have the basic position, are provided out of which they can execute brake-pressure build-up and reduction strokes. For the rear-wheel brakes brake-pressure regulating members of that type in which the basic position of the pistons corresponds to the largest volume of the outlet-pressure spaces of these brake-pressure regulating members are provided. These rear-wheel brake regulating members are utilized for brake-pressure generation and are shut off relative to the brake unit of the brake system even in the event of normal braking not subjected to anti-lock control. Furthermore, at least one sensor generates an output signal which is fed to the electronic control unit so that, in the event of braking initiated by the driver and continuing to be controlled, is a measure of the expected value (driver's wish) of the vehicle deceleration. The electronic control unit is configured so that, from a comparative processing of the signals characteristic of the piston positions of the brake-pressure regulating members of the front-wheel brakes with a signal characteristic of the expected value of the vehicle deceleration, it generates activating signals for the drives of the brake-pressure regulating members of the front-wheel brakes with the effect of the follow-up of their pistons into the position assigned to the expected value.
Included among the advantageous functional properties of the brake-pressure control device according to the present invention is the brake-pressure control device allowing an automatic activation of the brakes both of the driven and of the non-driven vehicle wheels. This activation is a precondition for a "complete" control of the driving dynamics, and the possibility of automatically controlled full braking, the need for which can be recognized from the sensor output signal characteristic of the driver's wish, for example as a result of a time-differentiating processing of this signal. Thus, when the driver actuates the brake pedal hastily, full braking with a maximum pressure-rise rate can already be initiated when the actuating speed exceeds a threshold value. In this situation, the inlet valves are closed and the brake pressures are initially generated solely by the brake-pressure regulating members.
The brake-pressure regulating members can also be used in the manner of a parking brake such that, in slow-moving traffic requiring frequent stopping and starting again after braking in which the vehicle has come to a standstill, the inlet valves are held closed without the driver having his foot on the pedal, until the driver actuates the accelerator pedal again. The brakes are relieved of pressure to the reservoir via the non-actuating master cylinder of the brake system.
Because a considerable part of the brake-fluid quantity displaced towards the wheel brakes is fed into the wheel brakes by the brake-pressure regulating members, for normal braking operation a markedly reduced pedal travel and, therefore, a sensitive reaction behavior of the brake pedal are obtained, with the driver being informed directly of the pressure prevailing in the wheel brakes.
Due to the property of the brake-pressure control device according to the present invention that the brake-fluid quantity, m.sub.S, which can be forced into the front-wheel brakes by the brake-pressure regulating members, is controlled electronically, a ratio, m.sub.S /m.sub.B, constituting the brake fluid quantity, m.sub.S, to the brake-fluid quantity forced into the front-wheel brakes as a result of the actuation of the master cylinder, m.sub.B, can be adjusted so that, in principle, any pedal-travel/brake-pressure characteristics can be preset by the configuration of the electronic control unit and, if appropriate, can be changed at the request of the vehicle owner.
Adjustability in this regard can be effected in a simple way by configuring the electronic control unit so a weighting factor adjustment effects adjustment of the ratio of brake-fluid quantity forced into the front-wheel brakes via the associated brake-pressure regulating members to the brake-fluid quantity forced into the front-wheel brakes as a result of actuation of a master brake cylinder. As a result, the sensor output signal representing the driver's wish is comparable with the position actual-value signal of the position sensor monitoring the piston position of the respective brake-pressure regulating member of the front-wheel brakes. This also allows the drive control of the brake-pressure regulating members to be implemented in the manner of a very "rapid" sensitive analog control which can also be effected by the simplest possible circuitry, for example by activating the electromotive drives of the brake-pressure regulating members by an activating signal corresponding in amount and sign to the "difference" between the pedal-position-sensor signal and the piston-position-sensor signal.
By the utilization of further measures in which the weighting factor for the signal representing the driver's wish can be varied as a function of the sensor output signal characteristic of the position of the piston of the brake-pressure regulating member and the variation of the weighting factor for the sensor signal representing the driver's wish takes place with an increasing pressure build-up displacement of the pistons of the brake-pressure regulating members of the front-wheel brakes, with the effect of a reduction or appropriately modified, correspondingly diverse and more or less progressive pedal-travel/brake-pressure characteristics can be achieved.
The brake-pressure control device is configured such that the brake-pressure regulating members of the wheel brakes, for example front-wheel brakes, respectively combined in a brake circuit have a self-locking drive, characterized in that the main brake line of this brake circuit, the main brake line being connected to the brake unit, can be shut off relative to the brake unit by means of an electrically activatable change-over valve, and in that there are provided non-return valves which are connected parallel to the inlet valves of this brake circuit and are loaded in an opening direction by a relatively higher pressure in the wheel brakes of the brake circuit than at a brake-side connection of the change-over valve. With the change-over valve opened, a brake-pressure reduction via non-return valves can be achieved, even when the current supply for the regulating-member drives has failed and also should at least one of the associated inlet valves remain in its shut-off position as a result of a malfunction. For a control of driving dynamics, the change-over valve is closed.
Since an amount of the brake pressure which corresponds to the amount necessary for obtaining a maximum vehicle deceleration can be built up in the individual wheel brakes by the brake-pressure regulating members, the brake-pressure regulating members as a whole can also be utilized as a "hydraulic brake booster", so that it is possible to have the configurations of the brake system of the vehicle in which the brake unit is a static dual-circuit master cylinder which can be actuated directly by means of the brake pedal of the brake system or as a single-circuit master cylinder which can be actuated directly by means of the brake pedal of the brake system and to which are connected the front-wheel brakes combined to form a front-axle brake circuit. Thereby, even in the event of a failure of the brake-pressure regulating members, a sufficient minimum deceleration of the vehicle can still be achieved.
Alternatively, the brake-pressure control device can be configured with brake-pressure regulating members as pressure modulators and with inlet valves assigned individually to the rear-wheel brakes. The brake-pressure regulating members can also be configured in the way to dispense with the need for inlet valves.
In combination with the foregoing, the compensating channels can be configured as simple snifting holes, because the function of the brake-pressure regulating members guarantees that piston gaskets are not exposed to any appreciable pressure when they run over the snifting holes.
The advantage of having non-self-locking electromotive drives of the brake-pressure regulating members is that non-return safety valves otherwise necessary are not required.
A pressure sensor which records the pressure generated in the master cylinder, with its output signal being a measure of the force with which the driver actuates the brake pedal, is an additional way for reliably recognizing the driver's wish with regard to the development in time of the vehicle deceleration.
Pressure sensors provided for at least one of the rear-wheel brakes and/or at least one of the front-wheel brakes improve the reliability of an electronic control of the brake-force distribution and/or of a control of driving dynamics.